Previous work from this laboratory has demonstrated that the Harvey and Kirsten sarcoma viruses arose through the recombination of a mouse leukemia virus and endogenous rat genetic information. It has been the aim of this investigation to determine what percentage of the Harvey and Kirsten sarcoma viral information is represented as rat information vs mouse genetic information and where each set of genetic material is located in the viral genome. This information will allow a greater understanding of the mechanism of formation of these sarcoma viruses and how they transform cells. BIBLIOGRAPHIC REFERENCE: Young, H.A., Shih, T.Y., Scolnick, E.M. and Parks, W.P.: Steroid induction of mouse mammary tumor virus: Effect upon synthesis and degradation of viral RNA. J. Virol. 21: 139-146, 1977.